8.2 Exchanging information with BGP-3

8.2 Exchanging information with BGP-3

This document suggests the following guidelines for exchanging
routing information between BGP-4 and BGP-3.

To provide for graceful migration, a BGP speaker may participate in
BGP-3, as well as in BGP-4. Thus, a BGP speaker may receive IP
reachability information by means of BGP-3, as well as by means of
BGP-4.

A BGP speaker may inject the information received by BGP-4 into BGP-3
as follows.

If an AS_PATH attribute of a BGP-4 route carries AS_SET path
segments, then the AS_PATH attribute of the BGP-3 route shall be
constructed by treating the AS_SET segments as AS_SEQUENCE segments,
with the resulting AS_PATH being a single AS_SEQUENCE. While this
procedure loses set/sequence information, it doesn't affect
protection for routing loops suppression, but may affect policies, if
the policies are based on the content or ordering of the AS_PATH
attribute.

While injecting BGP-4 derived NLRI into BGP-3, one needs to be aware
of the potential information explosion when a given IP prefix denotes
a set of consecutive A/B/C class networks. Injection of BGP-4 derived
NLRI that denotes IP subnets requires the BGP speaker to inject the
corresponding network into BGP-3. The local system shall provide
mechanisms to control the exchange of routing information between
BGP-3 and BGP-4. Specifically, a conformant implementation is
required to support all of the following options when injecting BGP-4
received routing information into BGP-3:

inject default only (0.0.0.0), no export of any other NLRI

allow controlled deaggregation, but only of specific routes;
allow export of non-aggregated NLRI

allow export of only non-aggregated NLRI

The exchange of routing information via BGP-3 between a BGP speaker
participating in BGP-4 and a pure BGP-3 speaker may occur only at
the autonomous system boundaries. Within a single autonomous system
BGP conversations between all the BGP speakers of that autonomous
system have to be either BGP-3 or BGP-4, but not a mixture.